A carbon dioxide laser is a gas laser, which was invented in 1964 by Kumar Patel of Bell Labs, and is known to be one of the highest power, continuous wave lasers available. A carbon dioxide laser produces a beam of infrared light with a principal wavelength of 9.4 or 10.6 micrometers. In the examples of the summary and detailed description the 10.6 micrometer (10,600 nanometer) wavelength is used. The gas used in commercially available carbon dioxide laser is typically about 10-20% carbon dioxide, 10-20% nitrogen, a few percent of hydrogen or xenon, and a remainder being helium. Special materials are used for the optics and mirrors of a carbon dioxide laser, which operates in the infrared spectrum. Mirrors may be made of coated silicon, molybdenum or gold, while windows and lenses may be made of germanium or zinc selenide. In early versions, lenses and windows were made of crystalline sodium chloride or potassium chloride. On one end of the laser a total reflector is placed and on an opposite end a partially reflective mirror or output coupler may be placed, providing for a continuous infrared laser beam. The percentage of infrared energy reflected may be in range from five to fifteen percent, typically. Edge coupling may be used to reduce optical heating. Power of a carbon dioxide laser may be selected from milliwatts to hundreds of kilowatts in continuous wave setups, but output power may be gigawatts in a q-switched setup. A modulator may be used to externally trigger a q-switched carbon dioxide laser. For this reason, commercially available carbon dioxide lasers are used for industrial cutting and welding applications and for surgical lasers, to a lesser extent. Military uses of carbon dioxide lasers are limited to range finders, laser designators, and laser detection and ranging applications (LADAR). The trend is to replace carbon dioxide lasers with solid state lasers that are more compact, more robust and easier to maintain in a field environment.
Near-infrared and ultraviolet wavelengths may be dangerous for eye safety at high power and short ranges, generally, because there are no visual cues that warn a person that high intensity light is impinging on the retina, as there would be in a visible laser light. Visible laser light may dazzle a person, but it is unlikely to damage the retina, unless high power is used. However, ultraviolet wavelengths may be easily shielded using glass or plastic barriers. One of the concerns for military uses is that the observation of the battlefield using binoculars, scopes or other magnifying devices may collimate and/or focus laser light, which increases ocular damage. The lenses of such devices shield the viewer from some ultraviolet radiation and virtually all of the beam emitted from a carbon dioxide laser. Thus, standards for “eye safe” laser power and wavelength have been established and adopted by the military use and standards setting bodies for which designers of laser emitting systems must be cognizant, such as FDA/CDRH21 CFR 1040 Performance Standard for Light-Emitting Products. “Eye safe” means the power and wavelength of a laser considered “eye safe” by FDA/CDRH21 CFR 1040 and military specifications, depending on the specific application for which a system is intended for use. Thus a laser system for use as a stand-off detector in urban and rural environments must meet eye safe requirements of the military, for military applications such as a detection of hazardous chemical and explosives residue. Infrared and far infrared may damage a cornea, but do not focus on the retina, due to their longer wavelengths. Thus, infrared lasers are considered to be safer than visible light, provided that the power and intensity of the beam are within safe limits. However, infrared wavelengths are difficult to achieve, while maintaining beam output power, using solid state lasers that are preferred in commercial and military applications. Thus, eye safe lasers are considered to have disadvantages that have limited or prevented their use in laser induced plasma spectroscopy.
Nd:YAG lasers are known that emit light in the ultraviolet wavelengths. Nd:YAG lasers may be frequency shifted, have high power outputs, may be pumped using stable, longlasting laser diodes or Diode Pumped Solid State (DPSS) lasers, and are suitable for use in applications that require a hardened, robust laser system. Nd:YAG lasers are the most common lasers used in laser range finding and have virtually replaced carbon dioxide lasers in military and civilian laser range finding. Pulsed Nd:YAG lasers are commonly used for cutting and welding applications for steels and superalloys at a power in the range of 1 to 5 kilowatts, replacing carbon dioxide lasers for all but the highest power applications. In q-switched Nd:YAG lasers, power outputs of 20 megawatts may be obtained. Very compact devices are affordable and are used for applications from golf to the home improvements. The output wavelength of Nd:YAG lasers, without frequency shifting, is 1064 nanometers, but there are also transitions near 940, 1120, 1320 and 1440 nanometers. Frequency shifting and/or addition provides green, blue and yellow visible light output, as well. However, it is important to avoid wavelengths from 400 to 1400 nanometers due to the magnification attributed to the human cornea focusing these wavelengths on the retina, resulting in permanent damage to the retina, especially at high power. For example, FIG. 22 illustrates a relationship between wavelength and a maximum eye safe laser power for a 50 millimeter diameter laser beam (collimated or incident diameter at the cornea).
Er:YAG lasers are known that emit light in the infrared with a wavelength of 2490 nanometers. Er:YAG lasers are often used as surgical lasers, because the energy is readily usable for heating water molecules. The compact size, electronic integration, stability and robustness of these solid states lasers have displaced much of the use of carbon dioxide lasers, which are frequently relegated to applications that require high power beams for cutting and welding, for example.
Frequency doubled Nd:YAG lasers have a wavelength of 532 nanometers (green light), which may be used in certain laser eye surgeries. Frequency doubled and tripled Nd:YAG lasers may be used for specific applications. Frequency shifting may be accomplished using nonlinear optical materials, such as lithium triborate.
A plasma is one of the phases or states of matter. The others are solid, liquid and gas. A plasma is defined as the state of matter in which electrons are dissociated from the nucleus of an element. Thus, a plasma is sometimes referred to as an ionized gas, although the two states of matter are independent from one another. Lightning is known to create a plasma. Even a spark of sufficient intensity is capable of generating a plasma.
Light and other electromagnetic radiation are capable of energizing electrons. For example, radio waves (a type of electromagnetic radiation) are produced by the effect of an alternating currents on electrons in a conductor acting as a transmitting antenna. They are detected by the effect of the radio waves on electrons in a receiving antenna. The effect of various wavelengths of energy on the energizing of electrons in conductors is well known; however, it is less well known how to energize electrons in a plasma.
Pulsed plasma spectroscopy using a pulsed laser to ablate material from a surface is known. However, the laser power required is high, requiring a laser with sufficient energy, focused on the surface at such intensity, that a significant amount of the surface material is ablated. In some applications, it is preferred to leave a surface unmarred by the inducement of a plasma. Also, high power lasers are an eye hazard, which must be avoided in actual practice, unless eye safe frequencies of the emitted laser beam are used. A range of eye safe wavelengths for laser light are known. Retinal damage is the most severe form of eye damage, which must be avoided at all cost in an open field environment. Thus, certain wavelengths that are not capable of causing retinal damage are preferred. In some circumstances, wavelengths in the ultraviolet range are acceptable as eye safe wavelengths, because any damage may be avoided by either limiting the power output of the laser or shielding vision using a clear glass or polymer barrier. One specific hazard is encountered when lasers are used on a battlefield. Anyone observing the battlefield using optical magnification, such as binoculars, is even more susceptible to eye damage from some laser wavelengths. Laser wavelengths in the ultraviolet are attentuated by the lenses in binoculars. Thus, ultraviolet or longer wavelengths are preferred for military applications of laser spectroscopy systems.
Laser-Induced Breakdown Spectroscopy (LIBS) is one example of a plasma spectroscopy. LIBS is also referred to as Laser Spark Spectroscopy (LASS) or Laser-Induced Plasma Spectroscopy (LIPS). The technique was first developed at Los Alamos National Laboratories and involves focusing a laser pulse onto a surface. The energy from the pulse heats, vaporizes, atomizes and then ionizes the material on the surface, resulting in a small, hot plasma. The atoms and ions in the plasma emit light which is then detected and analyzed. Each element has a unique spectral signature, which allows each of the elements in the plasma to be identified. This technique has been applied to the rapid analysis of metals for the purpose of sorting and/or monitoring composition during processing. It has been proposed that a LIBS unit could be fit on a military vehicle or a man portable device for use as a detector for land mines and the like; however, it is believed that no practical device has been tested in the field. One specific shortcoming of known systems is that the laser light is produced using a Nd:YAG laser at high power and at a wavelength that is not eye safe. Frequency shifting of the Nd:YAG laser may be used to shift the output of the laser into the ultraviolet range, but we do not know of any system that has successfully demonstrated a successful test of such a system at a stand off distance of twenty meters or greater. Preferably, any LIBS, LIPS or LASS system would operate at an eye safe wavelength and at a stand off distance of up to 50 meters or greater to reduce the chance of injury during detection of explosives, for example. No known system has demonstrated a fifty meter stand off distance at an eye safe wavelength at least in the ultraviolet range.
The main shortcomings of such systems is the need for a high power laser that increases the power consumption, creates a potential for severe eye damage, and requires impractical sensitivity of the collection optics required for a practical device, including frequent tuning and calibration, for example. In order to achieve a highly focused beam directly on the surface of a target for generating the required plasma with a strong enough signal to detect at any reasonable stand-off distance from the source of the laser, all of the shortcomings of LIBS, LIPS and LASS are present or must be accounted for. Safety of such a laser system is a primary concern, since high power is needed and laser wavelengths that create an eye hazard are typically used for generating the plasma. Lasers are known to cause damage to eyes, and even reflected radiation of high power devices such as required in LIBS may be hazardous to vision.
The Townsend Effect was first described by Sir John Townsend in 1915 in his book Electricity in Gases (Oxford University Press, London). In his example, he used a low pressure chamber to contain a gas, X-rays to generate a plasma, and parallel plates to generate an electric field. Townsend realized that there was a mathematical correlation between amplification of the plasma, the gas pressure, and the magnitude of the electric field generated by the parallel plates. It is believed that nobody has previously appreciated that the Townsend Effect may be used to amplify the signal of a plasma created using a laser, such as in LIBS, and no use of the Townsend Effect has been made at a stand off distance from an excitation and amplification source.